Stress Relieving Semiconductor Layer

ABSTRACT

A semiconductor structure, such as a group III nitride-based semiconductor structure is provided. The semiconductor structure includes a cavity containing semiconductor layer. The cavity containing semiconductor layer can have a thickness greater than two monolayers and a multiple cavities. The cavities can have a characteristic size of at least one nanometer and a characteristic separation of at least five nanometers.

REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of co-pending U.S.Provisional Application No. 61/817,970, titled “Stress RelievingSemiconductor Layer,” which was filed on 1 May 2013, and which is herebyincorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to growth of semiconductor layers, andmore particularly, to an improved solution for growing semiconductorlayers.

BACKGROUND ART

Group III nitride semiconductors are widely used for efficient blue andultraviolet light emitting diodes, lasers, ultraviolet detectors, andfield effect transistors. Due to a wide band-gap, these materials areone of the primary choices for fabricating deep ultraviolet lightemitting diodes (DUV LEDs). In recent years, significant advances havebeen made in improving the efficiency of DUV LED. However, overallefficiencies of these devices remain low. For fabrication of DUV LEDs,it is crucial to achieve a high quality AlN buffer layer as anunderlying layer for the subsequent growth of any Al-rich group IIInitride semiconductor layers. However, it is extremely difficult to growan AlN layer with high crystal quality on sapphire, silicon carbide(SiC), and silicon, which are currently the main substrates for growthof group III nitride devices.

For light emitting devices, such as light emitting diodes (LEDs) andespecially deep ultraviolet LEDs (DUV LEDs), minimizing a dislocationdensity and a number of cracks in the semiconductor layers increases theefficiency of the device. To this extent, several approaches have soughtto grow low-defect semiconductor layers on patterned substrates. Theseapproaches typically rely on reducing stresses present in epitaxiallygrown semiconductor layers.

For example, one approach to reduce stress accumulation in anepitaxially grown layer relies on patterning the underlying substrateusing microchannel epitaxy (MCE). Using MCE, a narrow channel is used asa nucleation center containing low defect information from thesubstrate. An opening in a mask acts as a microchannel, which transferscrystal information to the overgrown layer, while the mask preventsdislocations from transferring to the overgrown layer. As a result, theovergrown layer can become dislocation free. The three-dimensionalstructure of the MCE also provides another advantage to stress release.The residual stress can be released effectively since the overgrownlayer easily deforms. In another approach, a mask is applied at alocation of a large concentration of dislocation densities to blocktheir further propagation.

Other approaches rely on epitaxially growing a group III nitride basedsemiconductor superlattice. The superlattice structure mitigates thestrain difference between an aluminum nitride (AlN)/sapphire templateand the subsequent thick Al_(x)Ga_(1-x)N (where 0≦x≦1) layers. Fordevices such as DUV LEDs, thick AlGaN epitaxial layers (e.g., of theorder of a few micrometers) are desirable to reduce current crowding.Using a superlattice approach, an AlN/AlGaN superlattice was grown toreduce biaxial tensile strain and a 3.0-μm-thick Al_(0.2)Ga_(0.8)N wasachieved on sapphire without any cracks. Such a superlattice can be usedto minimize the dislocation density due to varying stresses in thesublayers of the superlattice elements.

While the superlattice approaches allow some control of tensile andcompressive stresses in epitaxially grown nitride semiconductor layers,the approaches do not enable epitaxial growth of nitride basedsemiconductor layers with uniform composition. Based on previousexperience obtained from GaN growth, lateral epitaxial overgrowth (LEO)has been shown to be an efficient way for significant reduction ofdislocation in GaN films. Several other technologies evolved from LEO,such as pendeo-epitaxial, cantilever epitaxy, and facet controlled LEO,have also been developed. While the above approaches work well forepitaxial growth of GaN semiconductor layers, epitaxial growth of AlNlayers is more challenging due to small lateral growth of AlN films.

Another leading approach includes growth of AlN films over patternedsubstrates, such as, for example, patterned sapphire substrate (PSS).While the PSS based approaches generally produce an AlN layer withreduced stress and low dislocation densities, the patterning process andsubsequent growth of AlN films is technologically complicated andcostly.

One previously described approach proposes to reduce dislocation densitybetween an AlGaN layer and a sapphire substrate by forming aself-organizing porous AlN layer of non-coalescing column-like islandswith flat tops on the substrate. In this approach, the self-organizingporous AlN layer is grown at a relatively low V/III ratio of sixty.While the AlN layer is described as being porous, the disclosure doesnot define any details of such layer, nor does it disclose how the poresare connected or their sizes and densities.

SUMMARY OF THE INVENTION

The inventors propose a growth technology for a group III-V layer, inwhich the layer is grown at a significantly higher V/III ratio than thatdisclosed in the prior art. The inventors propose to control the sizeand density of cavities in the layer through a variable V/III ratioand/or a variable temperature schedule for growth of a nucleation layer,a cavity layer, and a subsequent semiconductor layer containing nocavities. Attributes of an embodiment of the cavity sizes and densitiesare described herein, and experimentally achieved results are presentedin which the cavities are well separated from one another and aremorphologically dissimilar to pores described in the prior art.

Aspects of the invention provide a semiconductor structure, such as agroup III nitride-based semiconductor structure. The semiconductorstructure includes a cavity containing semiconductor layer. The cavitycontaining semiconductor layer can have a thickness greater than twomonolayers and a multiple cavities. The cavities can have acharacteristic size of at least one nanometer and a characteristicseparation of at least five nanometers.

A first aspect of the invention provides a structure comprising: acavity containing layer, wherein the cavity containing layer is formedof a semiconductor material, has a thickness greater than twomonolayers, and has a plurality of cavities, and wherein the pluralityof cavities have a characteristic size of at least one nanometer and acharacteristic separation of at least five nanometers.

A second aspect of the invention provides a device comprising: a cavitycontaining layer, wherein the cavity containing layer is formed of asemiconductor material, has a thickness greater than two monolayers, andhas a plurality of cavities, and wherein the plurality of cavities havea characteristic size of at least one nanometer; and a semiconductorlayer immediately adjacent to the cavity containing layer, wherein thesemiconductor layer contains no cavities.

A third aspect of the invention provides a method comprising:fabricating a semiconductor structure, wherein the fabricating includesforming a cavity containing layer, wherein the cavity containing layerhas a thickness greater than two monolayers and a plurality of cavities,and wherein the plurality of cavities have a characteristic size of atleast one nanometer and a characteristic separation of at least fivenanometers.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 illustrates a conventional approach for reducing dislocationsaccording to the prior art.

FIG. 2 shows an illustrative structure according to an embodiment.

FIG. 3 shows an illustrative structure according to an embodiment.

FIG. 4 shows an illustrative structure according to an embodiment.

FIG. 5 shows an illustrative structure according to an embodiment.

FIGS. 6A-6C show illustrative structures including superlatticesaccording to embodiments.

FIGS. 7A and 7B show illustrative structures according to embodiments.

FIG. 8 shows an image surface morphology of an illustrative cavitycontaining layer according to an embodiment.

FIG. 9 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a semiconductorstructure, such as a group III nitride-based semiconductor structure.The semiconductor structure includes a cavity containing semiconductorlayer. The cavity containing semiconductor layer can have a thicknessgreater than two monolayers and a multiple cavities. The cavities canhave a characteristic size of at least one nanometer and acharacteristic separation of at least five nanometers. The inventorshave found that embodiments of such a structure can be configured toreduce internal stresses, threading dislocations, and cracks in thesemiconductor layers. Additionally, embodiments can be used to controland/or release built up internal stresses within the structure. As usedherein, unless otherwise noted, the term “set” means one or more (i.e.,at least one) and the phrase “any solution” means any now known or laterdeveloped solution.

FIG. 1 illustrates a conventional approach for reducing dislocationsaccording to the prior art. In this approach, a structure 1 includes asubstrate 2 on which a nucleation layer 4 is grown. A semiconductor(e.g., coalescing) layer 8 is grown on the nucleation layer 4. Thenucleation layer 4 is configured to reduce a number of dislocations 9present in the semiconductor layer 8 as compared to the number ofdislocations 9 that would be present were the semiconductor layer 8grown directly on the substrate 2.

FIG. 2 shows an illustrative structure 10 according to an embodiment.The structure 10 is shown including a substrate 12, a nucleation layer14, a cavity containing layer 16, and a semiconductor (e.g., coalescing)layer 18. The cavity containing layer 16 can act as a stress-relievinglayer in the structure 10. As illustrated, the semiconductor layer 18can have significantly fewer dislocations 19 than present in thesemiconductor layer 8 (FIG. 1) of the prior art.

In an embodiment, the semiconductor layers 14, 16, 18 of the structure10 are formed of materials selected from the group III-V materialssystem. In a more particular embodiment, each layer 14, 16, 18 is formedof a group III nitride material. The substrate 12 can comprise anysuitable substrate material. Illustrative substrate materials for groupIII nitride layers include sapphire, silicon carbide, silicon, lithiumaluminate (γ-LiAlO₂), LiAlO₂, lithium gallate, spinel (MgAl₂O₄), galliumnitride (GaN), aluminum nitride (AlN), Aluminum oxynitride, zinc oxide(ZnO), and/or the like. Each layer 14, 16, 18 can have a thickness thatis larger than two monolayers. In an embodiment, a thickness of eachlayer 14, 16, 18 is within a range of approximately ten to approximatelyten thousand nanometers. A thickness of the cavity containing layer 16can be selected based on a thickness by which the islands will coalesce,which is highly dependent on the original separation distance ofnucleation islands in the nucleation layer 14. A thickness of thesemiconductor layer 18 can be selected to limit cracking. For example,the thickness of the semiconductor layer 18 can be selected to reducedislocation density by approximately an order of magnitude over that ofthe prior art. In a more specific illustrative embodiment: a thicknessfor the nucleation layer 14 is between approximately 5 nanometers andapproximately 100 nanometers; a thickness of the cavity containing layer16 is between approximately 1 micron and approximately 10 microns; and athickness of the semiconductor layer 18 is between approximately 1micron and approximately 4 microns.

In an embodiment, each of the layers 14, 16, 18 are epitaxially grown onthe substrate 12. For example, the nucleation layer 14 can beepitaxially grown directly on the substrate 12. Growth of the nucleationlayer 14 can include growth of a plurality of nucleation islands 14A,14B at a growth temperature and V/III ratio configured to result inlarge nucleation islands 14A, 14B. For example, the growth temperaturecan be in a range of approximately 700° C. and approximately 1050° C.,and the V/III ratio can be in a range between approximately 1000 andapproximately 20000. During growth of the nucleation layer 14, a typicalsize of the nucleation islands 14A, 14B can be controlled by adjusting aV/III ratio used during the growth. In an embodiment, the nucleationislands 14A, 14B have a relatively large lateral size of a few to a fewhundred nanometers (e.g., few tens of nanometers), with a similarseparation distance between the islands 14A, 14B.

The cavity containing layer 16 can be epitaxially grown directly on thenucleation layer 14. In an embodiment, growth of the cavity containinglayer 16 is configured to result in one or more cavities (voids) 16A,16B being present in the cavity containing layer 16. For example, growthof the cavity containing layer 16 can be performed at a relatively hightemperature, which can be approximately a few hundred degrees Celsiushigher than a temperature used for growth of the nucleation layer 14. Acharacteristic size of the nucleation islands 14A, 14B can control acharacteristic size and density of cavities 16A, 16B present in thecavity containing layer 16 due to a correlation between the size of thenucleation islands 14A, 14B and the spacing of cavities. For example, ascavities can form at some, but not necessarily all, boundaries ofnucleation islands 14A, 14B, larger nucleation islands 14A, 14B willcause the cavities to be further apart. By growing the cavity containinglayer 16 on a nucleation layer 14 with nucleation islands 14A, 14Bhaving large lateral sizes, a density of cavities 16A, 16B present inthe cavity containing layer 16 can be reduced, which can significantlyreduce a number of threading dislocations 19 present in thesemiconductor layer 18.

Furthermore, a temperature and/or V/III ratio used during growth of thecavity containing layer 16 can control a vertical size of the cavities16A, 16B, e.g., by altering a lateral growth rate of the islandformations in the cavity containing layer 16. The presence of thecavities 16A, 16B in the cavity containing layer 16 can reduce internalstresses, threading dislocations 19, and cracks in the semiconductorlayers of the structure 10.

Subsequently, the semiconductor layer 18 can be epitaxially growndirectly on the cavity containing layer 16 using any solution. To thisextent, growth of the semiconductor layer 18 can be performed at anygrowth temperature and V/III ratio, configured to result in a coalescedsemiconductor layer. In an embodiment, growth of the semiconductor layer18 uses a lower V/III ratio and a higher growth temperature than thoseused in growing the cavity containing layer 16. In an embodiment, thesemiconductor layer 18 has a different stoichiometry than the cavitycontaining layer 16. A structure 10 including the cavity containinglayer 16 with the cavities 16A, 16B can reduce stresses associated withheteroepitaxy, which would otherwise be present in the semiconductorlayer 18. As a result, the semiconductor layer 18 will have a reducednumber of dislocations 19 and cracks.

As described herein, an embodiment of a semiconductor structure 10 canbe fabricated by altering one or more growth conditions (e.g.,temperature, pressure, composition, etc.) during formation of thevarious layers 14, 16, 18. Following is a description of an illustrativemethod for fabricating a semiconductor structure 10 including layers 14,16, 18 formed of aluminum nitride (AlN)-based materials. While layers14, 16, 18 are described as being distinct layers, when the layers 14,16, 18 are formed of a uniform composition (AlN in the example below),the structure 10 can be considered as having a single layer formed usingthree distinct growth periods (each of which corresponds to the layers14, 16, 18). Regardless, it is understood that the example below is onlyillustrative, and a similar method (but with different ranges for thegrowth conditions) can be applied to fabricate a semiconductor structure10 including layers 14, 16, 18 formed of any combination of one or moretypes of compositions, such as any type ofAl_(x)In_(y)B_(z)Ga_(1-x-y-x)N, where 0≦x, y, z≦1 and 0≦x+y+z≦1, alloylayers. In an illustrative embodiment, the aluminum molar fraction, x,is at least 0.8.

Growth conditions for an AlN-based nucleation layer 14 can be configuredto induce three dimensional growth. For example, a growth temperaturefor the nucleation layer 14 can be in a range between approximately 600°C. and 1300° C. and a V/III ratio can be in a range betweenapproximately 100 and approximately 50000. Use of a higher V/III ratioenhances vertical growth rate and restrains lateral growth. To thisextent, the V/III ratio and/or time period for the growth can beadjusted to provide a target characteristic size of the islands in thenucleation layer 14. Additionally, formation of the nucleation layer 14can use dissimilar materials including, for example, indium nitride(InN), gallium nitride (GaN), aluminum nitride (AlN), silicon nitride(SiN), any of their alloys, and/or the like.

Growth conditions for an AlN-based cavity containing layer 16 can beconfigured to induce island growth and the presence of cavities withinthe cavity containing layer 16. For example, a growth temperature forthe cavity containing layer 16 can be in a range between approximately800° C. and 1500° C. and a V/III ratio can be more than approximately100, e.g., in a range between approximately 100 and approximately 10000.The V/III ratio can be adjusted based on a target lateral growth rate,which can adjust a vertical dimension of the cavities. Additionally,additional auxiliary agents (e.g., precursors), such as zinc oxide(ZnO), titanium nitride (TiN), SiN, gallium arsenide (GaAs), aluminumarsenide (AlAs), GaN, InN, and/or the like, can be used to inducecavities within the cavity containing layer 16, thereby altering theelastic properties of the cavity containing layer 16.

Growth conditions for an AlN-based semiconductor layer 18 can beconfigured to induce coalescence of nucleated islands in the cavitycontaining layer 16 into a single layer. For example, a growthtemperature for the semiconductor layer 18 can be in a range betweenapproximately 1100° C. and 1600° C. and a V/III ratio can be in a rangebetween approximately 1 and approximately 2500. Subsequent growth canresult in a fully coalesced, pit-free single-crystalline AlNsemiconductor layer. The semiconductor layer 18 can be grown to anydesired thickness, e.g., within a range of thicknesses between 1nanometer and 10000 nanometers. An addition of a small molar fraction ofgallium (e.g., one to five percent) within the semiconductor layer 18can increase the coalescence of the nucleated islands. In an embodiment,a molar fraction of aluminum in the semiconductor layer 18 differs fromthe molar fraction of aluminum in the cavity containing layer 16 by atleast two percent. The pressure used for growing the semiconductor layer18 can be under 100 torr, and can be kept constant for the entireduration of the growth. The pressure can be balanced by, for example,modifying the amount of hydrogen in the chamber.

Various embodiments of the structure 10 can be achieved by varying oneor more of the growth parameters, such as temperature, pressure, V/IIIratio, molar concentration of group III elements, and/or the like.Additionally, one or more layers 14, 16, 18 can be grown with a flux ofone or more additional precursors. Illustrative precursors include:trimethylaluminum, trimethylgallium, trimethylindium, triethylgallium,triethylboron, and/or the like. The precursor fluxes can be delivered atflow rate(s) of at least one micro-moles per minute.

FIG. 3 shows an illustrative structure 20 according to anotherembodiment. In this case, the structure 20 includes a nucleation layer24, which can be epitaxially grown on the substrate 12, and contain nolarge scale cavities. In an embodiment, the nucleation layer 24 has athickness of at least one nanometer. It is understood that thenucleation layer 24 may contain small scale cavities and inhomogeneitiesdue to the nucleation islands 24A, 24B forming the layer 24. As usedherein, small scale cavities refers to cavities having a characteristicsize (e.g., as determined by a largest lateral cross-section of thecavity) of approximately one nanometer or less.

A cavity containing layer 26 can be grown on the nucleation layer 24using growth parameters configured to cause the formation of large scalecavities 26A, 26B in the cavity containing layer 26. The cavitycontaining layer 26 can have any thickness, e.g., in a range betweenapproximately 10 nanometers and 10000 nanometers. The growth conditionsfor formation of large structures leading to cavity containing layer 26can be chosen to ensure three-dimensional growth, e.g., by having a highV/III ratio combined with a few hundred degrees Celsius increase intemperature as compared to the temperature used to grow the nucleationlayer 24. The structures are then allowed to coalesce by changing thegrowth condition to promote two-dimensional growth, which can beaccomplished by a reduction of the V/III ratio and a further increase ingrowth temperature (e.g., by several hundred degrees Celsius).

The cavities 26A, 26B can be of various sizes and have variousdistribution and densities throughout the cavity containing layer 26.For example, a characteristic size of the large scale cavities 26A, 26Bcan exceed approximately one nanometer. In a more particularillustrative embodiment, a characteristic size of the large scalecavities 26A, 26B can range between approximately 10 nanometers andapproximately 4000 nanometers. Furthermore, the cavities 26A, 26B canhave any of various characteristic separations. In an embodiment, acharacteristic separation of the cavities 26A, 26B (e.g., as determinedby a shortest distance between the edges of two adjacent cavities) canrange between approximately 5 nanometers and approximately 1 micron(e.g., approximately a few hundred nanometers in an illustrativeembodiment). Regardless, as described herein, the characteristic size ofthe cavities 26A, 26B present in the cavity containing layer 26 isdetermined by the growth parameters, such as growth temperature,pressure, V/III ratio, and/or the like, used during epitaxial growth ofthe cavity containing layer 26.

In an embodiment, the growth parameters differ between adjacent layersin the semiconductor structure 20. For example, each semiconductor layerin a semiconductor structure can be grown at a temperature at least fivepercent different from the temperature used during growth of theadjacent layer(s). Similarly, each semiconductor layer in asemiconductor structure can be grown at a V/III ratio at least fivepercent different from the temperature used during growth of theadjacent layer(s). One or more portions of the epitaxial growthdescribed herein can be reapplied and/or applied repeatedly to producevarious structures with semiconductor layers and/or superlattices formodifying elastic/plastic characteristics of the semiconductor layersand for releasing internal buildup stresses.

For example, FIG. 4 shows an illustrative structure 30 according tostill another embodiment. In this case, the structure 30 includes anucleation layer 34 grown directly on a substrate 12 and a cavitycontaining layer 36 grown directly on the nucleation layer 34. Asdescribed herein, growth conditions used during growth of the cavitycontaining layer 36 can be configured to cause a plurality of cavities36A to be present in the layer 36. As shown in FIG. 2, a layer grown onthe cavity containing layer 36 (e.g., the semiconductor layer 18) can begrown using growth conditions in which no large scale cavities will bepresent. Alternatively, as shown in FIG. 4, a second cavity containinglayer 38 is grown directly on the cavity containing layer 36 usinggrowth conditions configured to induce cavities 36A present in thecavity containing layer 36 (e.g., those present at the top of the cavitycontaining layer 36) to extend into the second cavity containing layer38. As a result, the second cavity containing layer 38 also includes aplurality of cavities 38A, which are substantially aligned with cavities36A present in the cavity containing layer 36. Such cavities 38A canvary in shape and/or size from the cavities 36A present in the cavitycontaining layer 36, e.g., due to use of one or more different growthconditions for the second cavity containing layer 38.

FIG. 5 shows still another illustrative structure 40 according to anembodiment. In this case, similar to the structure 30 shown in FIG. 4, anucleation layer 34 is located directly on a substrate 12, and a cavitycontaining layer 36 is located directly on the nucleation layer 34.However, the structure 40 also includes a second nucleation layer 44,which is grown on the cavity containing layer 36, and a second cavitycontaining layer 46, which is grown on the second nucleation layer 44.To this extent, the structure 40 includes multiple nucleation layers 34,44 and multiple cavity containing layers 36, 46. While the structure 40is shown including two nucleation layers 34, 44 and two cavitycontaining layers 36, 46, it is understood that a structure 40 caninclude any number of nucleation and/or cavity containing layers.

As illustrated, the cavities 36A, 46A present in the respective cavitycontaining layers 36, 46 can have different sizes, different densities,and/or the like, depending on the growth conditions used for the layers36, 46. Furthermore, the cavities 36A, 46A present in the respectivecavity containing layers 36, 46 may not have any particular alignmentand can be misaligned. In an embodiment, a structure 40 includes acavity containing layer, such as the cavity containing layer 46, withcavities 46A having one or more characteristic dimensions (e.g., lateralsize, vertical size, density, separation, and/or the like) that differsby at least five percent from the corresponding characteristicdimension(s) of the cavities 36A present in the previous cavitycontaining layer 36.

Since the presence of cavities 36A, 46A affects the elastic propertiesof the corresponding semiconductor layers 36, 46, a multilayeredsemiconductor structure 40 containing multiple semiconductor layers 36,46 with variable cavity sizes and densities can allow for control ofelastic properties of the semiconductor layers throughout the verticaldimension of the semiconductor structure 40. Additional control ofelastic properties of a semiconductor structure can be provided by asuperlattice of semiconductor layers, in which semiconductor layers withcavities are interchanged with semiconductor layers having no largescale cavities.

To this extent, FIGS. 6A-6C show illustrative structures 50A-50Cincluding superlattices 55A-55C, respectively, according to embodiments.Each structure 50A-50C is shown including a substrate 12 and anucleation layer 54 located thereon, each of which can be configured asdescribed herein. Each superlattice 55A-55C can be grown on thenucleation layer 54. Each superlattice 55A-55C includes cavitycontaining layers 56A-56C interchanged with semiconducting layers58A-58C having no large scale cavities. While the structures 50A-50C areshown as including superlattices 55A-55C, each with six layers, it isunderstood that a superlattice described herein can include any numberof layers. Additionally, it is understood that an embodiment can providestructures with similar layer configurations as shown in FIGS. 6A-6C,which are not part of a superlattice.

Regardless, a superlattice 55A-55C can have any of various possibleconfigurations. For example, as illustrated in FIG. 6A, each of thelayers 56A-56C, 58A-58C in the superlattice 55A can have substantiallysimilar vertical thicknesses. In an embodiment, the superlatticeincludes multiple pairs of layers, where each pair includes a cavitycontaining layer, such as layer 56A, and an adjacent semiconductinglayer 58A. One or more attributes of one or both layers in a pair canvary from one side of the superlattice to the other side of thesuperlattice. For example, in FIG. 6B, the superlattice 55B includescavity containing layers 56A-56C having variable thicknesses Hthroughout the superlattice 55B. In an embodiment, the thickness H ofeach cavity containing layer 56A-56C decreases with distance from thenucleation layer 54. Similarly, as shown in FIG. 6C, the superlattice55C includes semiconducting layers 58A-58C having variable thicknessesH₁ throughout the superlattice 55C. In an embodiment, the thickness H₁of each semiconducting layer 58A-58C increases with distance from thenucleation layer 54. In a more particular illustrative embodiment, thethicknesses H and/or H₁ of layers in adjacent pairs of layers differ byat least five percent.

The various layers described herein can be formed of a substantiallyuniform composition. Alternatively, a composition of one or more layerscan be altered during formation of the layer. For example, FIGS. 7A and7B show illustrative structures 60A, 60B according to embodiments. Eachstructure 60A, 60B is shown including a substrate 12 and a nucleationlayer 64 located thereon, each of which can be configured as describedherein. A series of layers can be grown on the nucleation layer 64, andinclude cavity containing layers 66A-66C interchanged withsemiconducting layers 68A-68C having no large scale cavities. In anembodiment, the series of layers 66A-66C, 68A-68C forms a superlattice.

In FIG. 7A, the semiconducting layers 68A-68C in the structure 60A havecompositions that differ from each other by at least one percent. In anembodiment, the compositions of the layers 68A-68C can result in bandgaps for the layers 68A-68C that differ from each other by at least twopercent. The composition of a group III nitride layer 68A-68C can bedetermined based on a molar fraction of aluminum in the layer 68A-68C.In an embodiment, the composition of the layers 68A-68C can be chosen tobe roughly an average between 1 and a molar fraction of an n-typecontact layer in the structure. For example, if the n-type contact layercontains a molar fraction of aluminum, x=0.6, then the averagecomposition of the superlattice layers 68A-68C can be about x=0.8, withthe composition of individual superlattice layers chosen to maintainsuch an average composition. For instance, the superlattice layers cancomprise Al_(0.65)Ga_(0.35)N layers interchanged with AlN of anappropriate thickness to yield an average composition of 0.8 within suchsuperlattice.

Furthermore, one or more of the layers 68A-68C can have a gradedcomposition in which a molar fraction of one or more elements in thelayer 68A-68C varies throughout the layer 68A-68C. For example, for agroup III nitride layer 68A-68C, an aluminum molar fraction can varybetween different layers 68A-68C and/or within a layer 68A-68C. To thisextent, formation of a group III nitride layer 68A-68C can include aseries of growth steps, with each growth step having at least one of: achange in V/III ratio or a change in growth temperature. In anembodiment, such change(s) differ from the V/III ratio and/or growthtemperature used in a previous growth step by at least one percent.Alternatively, growth of a group III nitride layer 68A-68C can use acontinuously changing (e.g., monotonic, pulsed, or periodic) V/III ratioand/or growth temperature. In an embodiment, a molar fraction of anelement, such as an aluminum molar fraction in a graded group IIInitride layer, differs by at least one percent from one side of thelayer to the opposing side of the layer. The grading can vary, forexample, from a high aluminum content closer to an AlN layer on a sideof the structure 60A to a low aluminum content on a side of thestructure 60A closer to an n-type contact layer. Similarly, in FIG. 7B,the cavity containing layers 66A-66C in the structure 60B can havecompositions that differ from each other by at least one percent and/orhave graded compositions. Furthermore, the cavities within each of thecavity containing layers 66A-66C can have a characteristic density(e.g., separation) and/or a characteristic size that differs from thoseof the cavities in the other cavity containing layers 66A-66C by atleast one percent.

While illustrative scenarios of varying attributes (e.g., thicknesses,compositions, cavities, etc.) are separately shown and described inFIGS. 6A-6C and 7A-7B. It is understood that a structure can include anycombination of varying attributes. Furthermore, the attributes can bevaried in an alternative manner to those shown in FIGS. 6A-6C and 7A-7B.For example, thicknesses can vary in an alternative direction, vary in anon-sequential manner, and/or the like. Regardless, the attributes ofthe various layers can be configured to achieve a target distribution ofelastic properties of the semiconductor layers in a structure, and as aresult, a target distribution of stresses throughout the structure.

The inclusion of one or more cavity containing layers in a semiconductorstructure can be combined with other known techniques for manipulatingstresses within semiconductor layers. For example, a structure describedherein can include semiconductor layers having alternating tensile andcompressive stresses combined with cavity containing layers. Tensile andcompressive stresses can be varied, for example, by adjusting a V/IIIratio used in growing the layers.

The inventors have formed and analyzed structures described herein. FIG.8 shows an image illustrating surface morphology of an illustrativecavity containing layer according to an embodiment. The cavitycontaining layer was epitaxially grown using a method described herein.The image was captured using an atomic force microscope (AFM) and showsa five micron by five micron area of the surface. As illustrated, theatomic step shows an extremely flat surface with an RMS of approximately1.0 Angstroms.

A structure described herein can be implemented as part of any ofvarious types of optoelectronic or electronic devices. An embodimentprovides a device formed using group III nitride semiconductor layers.In such a case, controlling stress characteristics can be important forreducing cracks and dislocations. In a more particular embodiment, thedevice is a light emitting device, such as a diode, in which the controlof stress and dislocations within and in proximity of the active layercan provide improved device operation and extended lifetime of thedevice. Illustrative devices include: a laser diode, a light emittingdiode, a photodiode, a deep ultraviolet light emitting diode, a highmobility electron transistor, a field effect transistor, a p-n diode, aSchottky diode, and/or the like.

In an embodiment, a structure described herein, such as the structure 10(FIG. 2) or the structure 20 (FIG. 3), is used as a template forsubsequent formation of any of various types of devices. In this case,the template can be formed and subsequently provided for growth of oneor more layers of any combination of various types of semiconductordevices on the template. To this extent, the formation of thesemiconductor device can include: epitaxial growth of a n-type contactlayer on the semiconductor structure (e.g., template); epitaxial growthof an active layer (e.g., including quantum wells and barriers) on then-type contact layer; and epitaxial growth of a p-type contact layer onthe active layer. In another embodiment, the substrate can be removed,e.g., by laser, along the cavity regions to create a structure offree-standing semiconductor layers.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the devices designedand fabricated as described herein (e.g., including one or more devicesfabricated using a semiconductor structure described herein). To thisextent, FIG. 9 shows an illustrative flow diagram for fabricating acircuit 126 according to an embodiment. Initially, a user can utilize adevice design system 110 to generate a device design 112 for asemiconductor device as described herein. The device design 112 cancomprise program code, which can be used by a device fabrication system114 to generate a set of physical devices 116 according to the featuresdefined by the device design 112. Similarly, the device design 112 canbe provided to a circuit design system 120 (e.g., as an availablecomponent for use in circuits), which a user can utilize to generate acircuit design 122 (e.g., by connecting one or more inputs and outputsto various devices included in a circuit). The circuit design 122 cancomprise program code that includes a device designed as describedherein. In any event, the circuit design 122 and/or one or more physicaldevices 116 can be provided to a circuit fabrication system 124, whichcan generate a physical circuit 126 according to the circuit design 122.The physical circuit 126 can include one or more devices 116 designed asdescribed herein.

In another embodiment, the invention provides a device design system 110for designing and/or a device fabrication system 114 for fabricating asemiconductor device 116 as described herein. In this case, the system110, 114 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thesemiconductor device 116 as described herein. Similarly, an embodimentof the invention provides a circuit design system 120 for designingand/or a circuit fabrication system 124 for fabricating a circuit 126that includes at least one device 116 designed and/or fabricated asdescribed herein. In this case, the system 120, 124 can comprise ageneral purpose computing device, which is programmed to implement amethod of designing and/or fabricating the circuit 126 including atleast one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 110 to generate thedevice design 112 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 110 for designing and/or a devicefabrication system 114 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A structure comprising: a cavity containinglayer, wherein the cavity containing layer is formed of a semiconductormaterial, has a thickness greater than two monolayers, and has aplurality of cavities, and wherein the plurality of cavities have acharacteristic size of at least one nanometer and a characteristicseparation of at least five nanometers.
 2. The structure of claim 1,further comprising: a substrate; and a nucleation layer located on thesubstrate, wherein the nucleation layer has a thickness of at least onenanometer and contains no large scale cavities, and wherein the cavitycontaining layer is located directly on the nucleation layer.
 3. Thestructure of claim 1, wherein the characteristic size is in a range ofapproximately ten nanometers to approximately four thousand nanometers.4. The structure of claim 1, further comprising a semiconductor layerimmediately adjacent to the cavity containing layer, wherein thesemiconductor layer contains no large scale cavities.
 5. The structureof claim 4, wherein the semiconductor layer is located on an opposingside of the cavity containing layer as a substrate.
 6. The structure ofclaim 4, wherein the cavity containing layer and the semiconductor layerare part of a superlattice of semiconductor layers including a pluralityof cavity containing layers interchanged with a plurality ofsemiconductor layers.
 7. The structure of claim 1, further comprising asecond cavity containing layer, wherein the second cavity containinglayer has a thickness greater than two monolayers and a plurality ofcavities, and wherein the plurality of cavities of the second cavitycontaining layer have a characteristic size of at least one nanometerand are separated from each other by at least five nanometers.
 8. Thestructure of claim 7, further comprising a nucleation layer locatedbetween the cavity containing layer and the second cavity containinglayer, wherein the nucleation layer has a thickness of at least onenanometer and contains no large scale cavities.
 9. A device comprising:a cavity containing layer, wherein the cavity containing layer is formedof a semiconductor material, has a thickness greater than twomonolayers, and has a plurality of cavities, and wherein the pluralityof cavities have a characteristic size of at least one nanometer; and asemiconductor layer immediately adjacent to the cavity containing layer,wherein the semiconductor layer contains no cavities.
 10. The device ofclaim 9, further comprising: a substrate; and a nucleation layer locatedon the substrate, wherein the nucleation layer has a thickness of atleast one nanometer and contains no large scale cavities, and whereinthe cavity containing layer is located directly on the nucleation layer.11. The device of claim 9, wherein the cavity containing layer and thesemiconductor layer are formed of a uniform composition.
 12. The deviceof claim 9, wherein the cavity containing layer and the semiconductorlayer are a part of a superlattice of semiconductor layers including aplurality of cavity containing layers interchanged with a plurality ofsemiconductor layers.
 13. The device of claim 12, wherein at least oneof: the plurality of cavity containing layers or the plurality ofsemiconductor layers have thicknesses that vary by at least five percentwithin the superlattice.
 14. The device of claim 13, wherein the atleast one of: the plurality of cavity containing layers or the pluralityof semiconductor layers have increasing thicknesses from a first side ofthe superlattice to a second side of the superlattice.
 15. The device ofclaim 13, wherein the device is configured to operate as one of: a laserdiode, a light emitting diode, a photodiode, a deep ultraviolet lightemitting diode, a high mobility electron transistor, a field effecttransistor, a p-n diode, or a Schottky diode.
 16. A method comprising:fabricating a semiconductor structure, wherein the fabricating includesforming a cavity containing layer, wherein the cavity containing layerhas a thickness greater than two monolayers and a plurality of cavities,and wherein the plurality of cavities have a characteristic size of atleast one nanometer and a characteristic separation of at least fivenanometers.
 17. The method of claim 16, wherein the fabricating furtherincludes growing a nucleation layer on a substrate, wherein thenucleation layer has a thickness of at least one nanometer and containsno large scale cavities, and wherein the cavity containing layer isgrown directly on the nucleation layer.
 18. The method of claim 16,wherein the fabricating further includes growing a semiconductor layeron the cavity containing layer, wherein the semiconductor layer containsno large scale cavities.
 19. The method of claim 16, further comprisingfabricating a device using the structure.
 20. The method of claim 16,wherein the cavity containing layer forming includes inducing theplurality of cavities using an auxiliary agent.